Method and plant for continuous cellulose digestion



Nov. 7, 1961 METHOD J. c. F. c. RICHTER 3,007,839

AND PLANT FOR CONTINUOUS CELLULOSE DIGESTION Filed July 8, 1959 United States Patent C 3,007,839 METHOD AND PLANT FOR CONTINUOUS CELLULOSE DIGESTION Johan C. F. C. Richter, St. Jean Cap Ferrat, Alpes Maritimes, France, assignor to Aktiebolaget Kamyr, Karlstad, Sweden, a company of Sweden Filed July 8, 1959, Ser. No. 825,764 Claims priority, application Sweden July 9, 1958 6 Claims. (Cl. 16217) The present invention relates to a method and a plant or apparatus for the liberation of cellulosic fiber material by its digestion with digesting liquor at a raised pressure and a raised temperature in a continuous digester.

In apparatus and processes of this type, it is known that the mechanical stresses upon the fiber material, which take place in connection with the mechanical discharge of the fiber material out of the digester and its transfer to a lower pressure, are detrimental to the quality of the pulp obtained, if the fiber material simultaneously is subjected to an attack of unspent digesting liquor of the full digesting temperature.

The object of the invention is to improve the pulp quality, in methods and plants, as referred to, by decreasing the possibility of a chemical attack during the discharge of the pulp from the digester; and the essential characterizing feature of the invention resides in that a part of the digesting liquor is withdrawn from the fiber material immediately before the same passes out through the digester outlet and in that this withdrawn digesting liquor is replaced by a weaker liquid which then accompanics the fiber material out through the digester outlet. Preferably, this weaker liquid consists of digesting liquor diluted by washwater and taken from a device for washing the digested fiber material, e. g., a Wash filter, disposed downstream from the digester.

Usually, in practicing this invention, liquid is of a temperature considerably digesting temperature and accordingly, in the digester outlet, the fiber material will desirably be mixed with a liquid which in comparison to the aforesaid withdrawn digesting liquor is weaker as to its content of chemicals as well as being cooler and, therefore, less detrimental than the digesting liquor in two respects.

It is preferred, according to one embodiment of the invention, to collect a withdrawn digesting liquor fracthe replacement lower than the Thus, from the first such withdrawn fraction, being of high temperature, it is possible by lowering the pressure to obtain low-pressure steam which advantageously may be used for steaming or preheating the fiber material supplied to the digester; whereas the second withdrawn fraction, which is at too low a temperature for satisfactorily giving off usable steam, may be used for other heating purposes.

The invention will be more fully described hereinbelow with reference to the accompanying drawing which diagrammatically shows a continuous digesting plant to which the invention is applied.

In the drawing, the numeral 11 designates a steam vessel in which wood chips or other comminuted cellulosic fiber materials supplied through a low-pressure charging apparatus 15, are heated by low pressure steam supplied through conduit 17, while the material is stirred and conveyed towards the outlet end of the steaming vessel by means of conveyer screw 19. These structures are all conventional, as will be understood.

A desired superatmospheric pressure, preferably about 1 atmosphere gauge, is maintained in vessel 11 as by means of an automatic control valve 21.

A suitable digesting liquor, for instance sodium lye, is supplied through the conduit 22. The mixture of chips and lye is transferred by means of a high-pressure charging apparatus 23 from the vessel 11 into a loop of circulating lye flushing the chips to the upper end of a standing cylindrical digester 25, as indicated. The chips are fed downwardly into the digester as by means of a motor-driven screw conveyer 27, and a large part of the lye is withdrawn through a cylindrical sieve 29 and by means of pump 31 is pumped through a heat exchanger 33, where it is heated. From there, the lye is returned to the charging apparatus 23 and carries chips charged into the same, with it to the digester.

A further heating of the mixture of chips and lye is eifected by means of a circulation system comprising two sieve girdles 35, 37 in the digester wall, a pump 39, a heat exchanger 41 and a return conduit 43 extending centrally in the digester and discharging, at about the center of the digester, the circulating lye taken out from the sieve girdles and heated in the heat exchanger 41. In the heat exchanger, the lye is heated to a temperature of approximately C.

The sieve girdles 35, 37 may be of the type disclosed in Patent No. 2,695,232, and the disclosure of that patent is hereby incorporated herein by reference. A conventional valve 35a is shown for connecting the sieve girdles 35, 37 individually, or one at a time, to the pump 39, as indicated, .and also as suggested in the aforesaid patent.

A hydraulic pressure of the order of 10 atmosphere gauge is maintained in the digester. During the slow and even downward movement of the fiber material through the digester, a reaction with the digesting liquor takes place, as is understood, so that the fiber material is completely digested when it reaches the lower end of the digester. There the material is fed down into a centrally located outlet 45 as by means of a motor-driven scraping or stirring device 47. Inserted in the conduit connected to the digester outlet is a throttling device 49 by means of which the material is conveyed into a blow tank 44 in which atmospheric pressure or a low superatmospheric pressure is maintained. The steam given off due to the pressure drop escapes through an outlet 46, whereas the pulp is discharged through a bottom outlet 48 and is carried through a conduit connected there to, and in which a valve 50 controlled by the level of the blow tank is inserted, to the pulp vat. 52 of a conventional rotary suction filter. The valve 50 may be of the type disclosed in Swedish Patent 137,071. On the rotary sieve drum 54 of said suction filter a pulp layer 56 is collected which is washed as by means of washwater, for instance, warm or cold fresh 'water supplied through conventional spray tubes 58. Part of the filtrate which consists of digesting liquor diluted with washwater is collected in a container 60 from which it is returned through a conduit 51 to the digester to be used therein in accordance with the invention and in a manner to be described in more detail hereinafter.

Shown as being inserted in the digester wall at the lower end of the digester is a sieve girdle consisting of a perforated or slitted cylindrical sieve plate 53 arranged flush with the cylindrical wall of the digester, as disclosed in the aforesaid U.S. patent. A space 55 for collection of sieved-off or withdrawn digesting liquor is arranged in a widened part of the digester wall exterior of said sieve plate. Said collecting space is shown as being connected by means of a conduit 57 to a steam separating device 59, preferably made as a cyclone, with the conduit 57 opening tangentially thereinto, as is understood.

The upper steam outlet of cyclone 59 is shown as being connected through the conduit 17 to the steaming vessel 11, and therefore, a comparatively low superatmospheric pressure is maintained in the cylone 59. When the sieved-off or-withdrawn digesting liquor enters the cyclone 59, it gives oif a great part of its heat content as steam which is used for heating the chips in the steaming vessel 11, as will be appreciated. The remainder of this withdrawn digesting liquor may be carried off at a reduced temperature through the outlet '61 and the conduit 62, in which a valve 63 controlled by the level of the cyclone 59 may be inserted, and, for example, to a plant for recovery of the chemicals.

Adjacent to and below the sieve girdle 53, a second similar sieve girdle 68 is shown as being inserted in the digester wall. The space 70 for collecting digesting liquor'in back of the sieve 68 is separated or hydraulically isolated from the space 55 and is shown as being connected by means of a conduit 72 to a second steam separating device which also preferably is made as a cyclone 74 having a tangential inlet of the conduit72. The steam outlet of the cyclone 74 is connected to a condenser 76 in which the steam formed in the cyclone 74 is condensed and used, for instance, for producing warm water or for some other heating purpose, such as preheating the digesting liquor supplied to the digester through the conduit 22. The digesting liquor discharged through the'bottom outlet of cyclone 74 is delivered by means of a pump 78 through a valve 80 controlled by the liquid level of the cyclone and through the conduit 82 to a plant for recovery of the chemicals of the digesting liquor. The cyclone 74 can be shut-off by means of the valves 84, 86 when there is no need to take steam out of the digesting liquor flowing through the conduit 72, or'when the liquor temperature is so low that the steam given off is of practically no value. In such a case, by opening the valves 88, 90 said fraction of the digesting liquor can be carried to a container 92 from which it is pumped directly into the conduit82, valve 80 also being controlled by the liquid level in container 92, as indicated. Thus, in said latter case the heat content of said second lye fraction is not utilized. If desired, the cyclone 74 can be omitted from the apparatus.

Shown as being inserted in the conduits 57 and 72 are conventional valves 94 and 6, respectively, which are suitably controlled in known manner by the quantity of liquid flowing thereto in such a manner as to maintain said quantity at a desired value.

The digesting liquor taken out through the sieve girdles 53, 68 is replaced by liquid taken from the container 60 by the conduit 51 and pumped into the digester by means of a high pressure pump 65. Connected to the pressure side of the pump is a vertically etxending conduit 67 which penetrates the digester wall close to the upper end of the digester, as shown, and runs concentrically within the tube 43, preferably being suspended along the vertical center line or axis of the digester and having an open lower end at about the level of the sieve :girdle 53. Preferably, the lower end of tube 67 is perforated, as shown.

Liquid pumped in through conduit 67 is distributed from the lower end thereof in the digester radially in all directions and displaces a corresponding quantity of digesting liquor. The digesting by the discharge from the tube 67 in the digester and withdrawn through the sieve girdle 53, forms a first undiluted fraction having a substantially constant temperature and lye concentration. Farther downstream in the digester, the washing liquid supplied by tube 67 reaches the digester wall and consequently the digesting liquor taken out from the sieve girdle 68 is diluted or partially diluted and forms a second but diluted lye fraction of a reduced temperature. By the separation of these two distinct fractions, it is possible to receive in the cyclone .59 a steam quantity corresponding to the temperature liquor, which is displaced 1 drop of the digesting liquor from practically the maximum cooking temperature of about 170 C. to about the steaming temperature C., and simultaneously to have a very thorough displacement of the digesting liquor and a considerable temperature drop at the digester outlet. Diluting liquid is supplied through the conduit 67 preferably in such a quantity that the temperature at the outlet 45 is reduced to about 100 to C. Inasmuch as the lye concentration is simultaneously considerably reduced, there is no risk that the mechanical stresses exerted upon the pulp by the scraper 47 and the throttle valve 49 will cause any injurious or harmful attack upon the fibers, and the pulp will therefore be of optimum quality.

Instead of using the conduit 67, or in addition thereto, the replacement liquid can be suppliedto the digester through a fluid distribution system or arrangement connected to the pump 65 and opening into the space between the sievegirdle 63-and the scraping and stirring device 47. Such a system is shown in the drawing as comprising an annular tube or fluid conduit 69 which is connected to a number of branch tubes 71 distributed around the digester and penetrating its wall, as indicated, with tube 69 being connected to the outlet of pump 65. The tubes 71 have orifices which are radially or somewhat tangentially disposed in the same direction relative to the digester wall and situated at some distance from the digester wall, as indicated. If desired, the quantity of liquid supplied through this system may be in excess of the quantity of digesting liquid withdrawn from the digester so that the pulp at the digester bottom will be diluted to a lower consistency, thereby facilitating its discharge through the outlet 45 and the throttle device 49.

'The above-described embodiment is merely an example of an application of the invention, and it will be appreciated that. the details can be modified in various respects. Thus, the sieve girdles 53, 68 may be divided either along an axial plane or a transverse plane, only parts thereof being used at any given time and the connected parts being shifted periodically. Of course, the replacement liquid can be more or less pure water and the heat content of the withdrawn or sieved-off liquid can be utilized for purposes other than those mentioned above. If desired, the replacement liquid, referred to, may be supplied through an axial tube connected to the center of the rotary scraper 47 at the bottom of the digester. In case the amount of replacement liquid obtainable from the vessel 60 does not suffice for effecting a desired degree of cooling at the outlet end of the digester, an additional amount may be recirculated through a branch line connecting conduit 82 to conduit 51.

What is claimed is:

l. A method of liberating cellulosic fiber material by digestion thereof with a digesting liquor at a raised pressure and a raised temperature in a continuous digester having an inlet and an outlet, said method being characterized in introducing a weaker liquid centrally in said digester flowing outwardly and axially downwardly with said digesting liquor to displace at least a part of the same from the fiber material, withdrawing the displaced liquor from the digester immediately before the fiber material passes out through the outlet of the digester, some of the withdrawn digesting liquor being undiluted while some of it is diluted by said weaker liquid, simultaneously and separately collecting said diluted and undiluted withdrawn fractions, and discharging said fiber material through said outlet together with at least a portion of said weaker liquid, at least a portion of said weaker liquid having previously to passed through the digester outlet together with fiber material.

2. A method according to claim 1, characterized in that the undiluted fraction of the withdrawn digesting liquor is at superatmospheric pressure and is subject, after being Withdrawn, to a lo er p s r giving off steam which is then used for steaming of the fiber material supplied to the digester.

3. Apparatus of the character described for continuously digesting cellulosic fiber material comprising: an upright cylindrical digester vessel having an upper inlet and a bottom outlet; means for delivering fibrous material and digesting liquor to said digester vessel through said inlet; means for washing the fibrous material downstream of said digester vessel; means for conveying fibrous material from said digester vessel outlet to said Washing means; agitating means arranged in said digester vessel upstream from and adjacent to said bottom outlet; means for conveying a mixture of digesting liquor and Wash Water from said washing means back to said digester vessel, said last-mentioned means including a conduit having an outlet and extending axially downwardly along the central region of the digester vessel, said conduit constructed and arranged to introduce the mixture of Wash water and digesting liquid outwardly and axially downwardly into said vessel; first means for Withdrawing substantially undiluted digesting liquor from said vessel disposed adjacent said conduit outlet but above the efiective path of fluid flow therefrom; and second means disposed adjacent to and below said conduit outlet for simultaneously withdrawing digesting liquor from said vessel diluted by said mixture of wash water and digesting liquor from said conduit whereby said first means provides a source of a high quality heat from said undiluted digesting liquor and said second means provides a source of low quality heat from the diluted mixture thereof.

4. Apparatus defined in claim 3 wherein said conduit outlet is constructed and arranged to provide a substantially conical dispersion of said Wash water and digester liquid.

5. Apparatus defined in claim 3 including a steam separating device operatively connected to said first means, said device being at a lower pressure than said digester whereby steam is given ofi therein, and means for conveying this steam given oil? in said separating device to preheat the fibrous material being delivered to said digester.

6. Apparatus defined in claim 5 including a second steam separating device operatively connected to said second means to recover the low quality steam from the diluted digesting liquid recovered therefrom.

References Cited in the file of this patent UNITED STATES PATENTS 2,360,779 Lang Oct. 17, 1944 21,695,232 Richter Nov. 23, 1954 2,876,098 Schandroch Mar. 3, 1959 2,920,697 Langen Jan. 12, 1960 

1. A METHOD OF LIBERATING CELLULOSIC FIBER MATERIAL BY DIGESTION THEREOF WITH A DIGESTING LIQUOR AT A RAISED PRESSURE AND A RAISED TEMPERATURE IN A COMTINUOUS DIGESTER HAVING AN INLET AND AN OUTLET, SAID METHOD BEING CHARACTERIZED IN INTRODUCING A WEAKER LIQUID CENTRALLY IN SAID DIGESTER FLOWING OUTWARDLY AND AXIALLY DOWNWARDLY WITH SAID DIGESTING LIQUOR TO DISPLACE AT LEAST A PART OF THE SAME FROM THE FIBER MATERIAL, WITHDRAWING THE DISPLACED LIQUOR FROM THE DIGESTER IMMEDIATELY BEFORE THE FIBER MATERIAL PASSES OUT THROUGH THE OUTLET OF THE DIGESTER, SOME OF THE WITHDRAWN DIGESTING LIQUOR BEING UNDILUTED WHILE SOME OF IT IS DILUTED BY SAID WEAKER LIQUID, SIMULTANEOUSLY AND SEPARATELY COLLECTING SAID DILUTED AND UNDILUTED WITHDRAWN FRACTIONS, AND DISCHARGING SAID FIBER MATERIAL THROUGH SAID OUTLET TOGETHER WITH AT LEAST A PORTION OF SAID WEAKER LIQUID, AT LEAST A PORTION OF SAID WEAKER LIQUID HAVING PREVIOUSLY TO PASSED THROUGH THE DIGESTER OUTLET TOGETHER WITH FIBER MATERIAL. 